1. Field of the Invention
This invention relates generally to apparatus for downhole placement in a wellbore, to control fluid flow through the wellbore. With further specificity, this invention relates to a type of flow control apparatus, particularly suitable for placement in injection wells (that is, wells in which the primary mode of operation is injecting of fluids into a downhole formation), and which can be cycled through different operating modes by application and release of annulus pressure at the surface of the well.
2. Related Art
Subterranean boreholes, referred to in this application as “wells,” have long been drilled for the production of oil and gas. Wells typically comprise a relatively large diameter tubular string, called casing, which is cemented in a borehole drilled into the earth, into which is run a relatively small diameter tubular string, called tubing, through which fluids either flow from the well, or are injected into the well. As a matter of terminology herein, the term “tubing” refers to the tubular string through which fluid flow (whether production or injection) occurs.
Wells are also used for injection purposes; that is, instead of flowing fluids (oil, gas, and formation water) out of a subsurface formation and up through the tubing to the surface, injection wells are used to inject fluids from the surface down the tubing into a subsurface formation. Fluid injection may be used as a means of disposing of produced water (which, in some cases, cannot be discharged into a surface environment, due to regulatory requirements), and/or as a means of oil and gas production enhancement, by providing a means of formation pressure support.
A fundamental requirement for injection wells is that some form of flow control apparatus be in place to prevent fluid flow from the formation upward into the tubing, when such flow is not desired. It is important to understand that such flow, known as “backflow” to distinguish it from injection, is at times necessary. For example, although fluids being injected (typically, produced water) is processed so as to reduce solids content, often some small amount of solids remain in the fluid stream. At very high injection rates (thousands of oil field barrels per day), such solids can cause problems by partially blocking the formation into which the fluids are being injected. This is noted by increases in required injection pressures. To reverse this undesired condition, the injection well may be permitted to backflow, so that fluids flowing out of the formation will carry with them the objectionable solids, thereby cleaning the injection path.
While fluid flow can be controlled from the surface of the well, problems arise. A wellbore many thousands of feet deep correspondingly has a fluid column many thousands of feet long. When fluid backflow is ongoing, stopping the fluid column at the surface (that is, near the end of its path of travel) can give rise to what is commonly known as the “water hammer” effect. For this and other reasons, particularly (although not exclusively) in injection wells, especially in a flowback condition, fluid flow is preferably controlled by a flow control apparatus near the source of the flow—that is, one located in the tubing downhole, near the bottom of the well.
Various devices for certain types of subsurface fluid flow control are well known in the art, commonly referred to as Surface Controlled Sub Surface Control Valves, or SCSSVs. Such valves are primarily for safety purposes, to provide a means of shutting in producing oil and gas wells if surface controls fail. SCSSVs are typically run in the tubing relatively near the surface of the well, on the order of 1000 feet deep or less, and are controlled by a hydraulic control line run in the tubing/casing annulus, from the valve to the surface. The known art SCSSV design cannot be selectively placed into operating modes which are either sensitive to fluid flow, or insensitive to fluid flow.
There is, therefore, a need for a fluid flow control apparatus adapted to incorporated into a tubing string and placed downhole in a well, if desired many thousands of feet from the surface, and which can be controlled by the surface application of and removal of pressure, whether on the annulus (that is, the annulus between the tubing and casing) of the well or pressure applied via a control line run from the surface downhole to the apparatus. Application and removal of pressure from the surface of the well permits the apparatus to be placed into one of two modes: a sensitive-to-flow mode, in which the apparatus opens in response to fluids being injected, thereby permitting fluid injection, but closes to prevent fluid backflow; and an insensitive-to-flow mode, in which the apparatus is open and permits fluid injection and backflow. The present invention therefore lends itself to operation either by annulus pressure or control line pressure.